Mixed Acid-Base Disorders: ABG Systematic Interpretation

Mixed acid-base disorders occur when two or more primary acid-base disturbances coexist simultaneously. A systematic approach to ABG interpretation is essential: Step 1: Assess pH (<7.35 = acidemia, >7.45 = alkalemia, 7.35-7.45 = normal or compensated). Step 2: Identify the primary disorder by matching pH direction with the abnormal component (if pH is low and PaCO2 is high = respiratory acidosis; if pH is low and HCO3 is low = metabolic acidosis). Step 3: Assess compensation - respiratory compensation for metabolic disorders is rapid (hours); renal compensation for respiratory disorders takes 3-5 days. Step 4: Calculate anion gap if metabolic acidosis is present (Na - Cl - HCO3; normal 8-12). Step 5: Calculate delta-delta if anion gap is elevated: Delta ratio = (change in AG)/(change in HCO3). If delta ratio >2: concurrent metabolic alkalosis; if <1: concurrent non-anion gap metabolic acidosis. Step 6: Apply compensation formulas to detect mixed disorders: if actual compensation differs significantly from expected, a second primary disorder is present. Common mixed disorders include: metabolic acidosis + respiratory acidosis (cardiac arrest), metabolic acidosis + metabolic alkalosis (DKA...